While we have shown that initial exposure to DNFB through tail skin leads to unresponsiveness rather than sensitization in C57BL/6 mice, painting DNFB on tails of BALB/c mice leads to sensitization. We shall examine a variety of inbred strains in this regard and correlate the findings with strain-specific densities in tail skin of various mouse strains. Neonatal tolerance of H-2 transplantation alloantigens will be studied by using several adoptive transfer protocols designed: (a) to determine whether a suppression mechanism is operative in these animals; (b) to identify it (them); and (c) to determine the immunogenetic basis for differences that we expect to find depending upon the type and degree of alloantigenic disparity. An analysis will be carried out to define why the testis, despite its abundant lymphatic drainage, provides a privileged site for allografts. MHC-incompatible parathyroid allografts placed in the testes of previously parathyroidectomized rat hosts will be used to test both primary alloreactivity and second-set responses in this site, as well as the role of the spleen in prolonging allograft survival. The passage of functional T cells from mother to fetuses and neonates will be addressed, using contact sensitization to DNFB. Foster-nursing experiments will be performed using females immunized during pregnancy or lactation, followed by skin testing of weanling animals for positive reactions.